This invention relates generally to radio frequency signal direction finding apparatus and more particularly to receivers used in such apparatus for detecting radio frequency signals using multibeam radio frequency array antennas.
As is known in the art, it is often desirable to determine the direction of a source of radio frequency signals. One techique used to determine such direction is through the use of a multibeam array antenna. As is known, such array antenna may be designed so that it produces a plurality of simultaneously existing beams of radio frequency energy, each one of such beams having the gain and bandwidth of the entire antenna aperture. One such multibeam array antenna is described in U.S. Pat. No. 3,761,936 issued Sept. 25, 1975, "Multi-Beam Array Antenna", inventors Donald H. Archer, Robert J. Prickett and Curtis P. Hartwig, assigned to the same assignee as the present invention. Such antenna includes an array of antenna elements, coupled to a microwave lens through constrained electrical paths. A desired number of simultaneous beams may be obtained, the constrained electrical paths and the microwave lens equalizing the time delay of received energy between a given one of a number of feed ports and all points on a corresponding planar wavefront of the received energy. However, because the beam associated with each one of the feed ports has finite width, the beams associated with adjacent feed ports overlap one another. Consequently, received radio frequency energy propagating along a particular wavefront appears not just at a single feed port but portions of such received energy appear at adjacent feed ports, the relative level of the signals received at the feed ports being related to the direction of the source of the radio frequency energy relative to the array antenna. Further, because any practical array antenna of the type described has a finite number of feed ports, each corresponding to a corresponding discrete angular direction, receive radio frequency energy may often be received along a direction between two adjacent ones of the discrete angular directions and therefore an interpolation process is needed to determine the direction of the received energy to an accuracy, or resolution, greater than the resolution provided by a finite number of feed ports.
One technique used to determine radio frequency energy source direction using a multibeam array antenna of the type described has been to pass the energy received at each of the feed ports through separate receiver channels, heterodyne the received radio frequency signals to a suitable video frequency signal, convert the level to such converted signals to digital numbers and digitally compute the angle or direction of the source by comparing the relative levels of the received signals. While such technique is adequate in some applications, in other applications as where it is necessary to compare signal levels having differences of a few db and signal level variations in the order of 40 db, maintaining the separate channels "matched" over such 40 db dynamic range is an extremely difficult process requiring sophisticated, costly equipment.
A second technique used is a time multiplexing arrangement. Here each feed port is sampled, fed through a single receiver channel, heterodyned to a video frequency and demultiplexed into separate signals, each representing the signal level at a corresponding one of the feed ports. The "interpolation" is made by comparing the relative amplitudes of the separate signals. Again, when the relative levels of the received signals may be as small as a few db an where the absolute value of the level of a received signal may vary over 40 db, such arrangement requires that the time multiplex switching means have extremely low noise characteristics and extremely accurate switching characteristics thereby requiring sophisticated, costly apparatus.